In recent years, there has been a rapid growth in the application of silicon-based electronics systems, especially for automotive applications. The silicon-based electronics are used to store control algorithms, process information, and to direct actuators to perform various functions, including steering, suspension, and display of driver information, to name but a few. While the electronics revolution unfolds, sensor technology, on the other hand, is lagging behind, and sensor designs continue to be based on dated technologies with associated limitations. Recent trends have identified silicon as a material well suited for future sensor technology which hopefully will close this technology gap and permit greater application of control systems utilizing sensor technology.
Existing control systems use silicon-based electronics, and nearly all have embedded microprocessors. Silicon is widely recognized in the industry as being suitable for this application in view of its high reliability, high strength and low cost. In addition, silicon sensor designs can be created using a variety of manufacturing processes one of the most promising of which is referred to as "micromachining" which uses chemical processes to introduce three-dimensional mechnical structures in silicon. These "microstructures", as they are called, can be made sensitive to specific physical phenomena, such as acceleration, gas flow rate or pressure, by taking advantage of the special piezo resistive properties of silicon. For example, a micromachined cantilivered beam produces a minute resistance change when flexed by the force of acceleration. However, the output signal from this micromachined sensor is very small (millivolts), so that additional electronic circuitry is necessary for signal conditioning and amplification. These electronic circuits are usually integrated circuit chips which are interconnected to the micromachined element. Different aspects of micromachining are reviewed in Lee et al, "Silicon Micromachining Technology for Automotive Applications", SAE Publication No. SP655, Feb. 1986, and the content of that publication is hereby incorporated by reference.
A disadvantage associated with polycrystalline silicon is that it is known to have an inherent high compressive stress. For example, undoped crystalline polysilicon has a stress of the order of -5.times.10.sup.9 dyne/cm.sup.2. This high compressive stress is disadvantageous especially when polysilicon is used for the fabrication of free-standing microstructures, such as cantilivers or bridges, which must be mechanically stable and not buckle or break. Such structures must have a low level of stress in order to facilitate the formation of free-standing structures of sufficient dimension to be useful as sensing elements. In a typical polysilicon deposition process used widely in the fabrication of integrated circuits today, silane gas is injected into a process tube at low pressure and a temperature of approximately 625.degree. C. These processing conditions produce a very uniform layer of deposited polysilicon material on a substrate. However, thermal coefficient of expansion differences of the polysilicon layer and the underlying material will produce a net compressive force in the polysilicon and this gives rise to the disadvantages noted earlier.
Recently, there has been much research into methods for producing stress-free polysilicon. These methods have primarily been to deposit the silicon at a temperature that will produce an amorphous silicon film having little or no crystalline structure present. There have been other attempts to anneal the polysilicon in different ways to relieve the stress. All of the prior methods suffer from the disadvantage that they change the polysilicon deposition parameters and utilize high temperatures (i.e. above 600.degree. C.), and so are incompatible with current technology trends and processing methods. In particular, the use of high temperatures for annealing and other processing is precluded because of the potential heat damage to the electronic components.